Radar systems are finding increased use in the motor vehicle sector for the purpose of rendering assistance to advanced driver assistance systems of the motor vehicles. One of the main tasks of said radar systems consists of ascertaining a distance from objects as well as a velocity of the objects (e.g., vehicles, pedestrians, stationary obstacles, etc.) in the vicinity of the motor vehicle. This is important for the adaptive cruise control (ACC) driver assistance system, for example, where a precise estimate of a distance and a relative velocity of the vehicle are used for ascertaining appropriate actions of the motor vehicle.
In motor vehicles, said radar systems may also be used for the realization of safety functions, such as warning the driver in critical situations and initiating a full application of the brakes if a collision can no longer be avoided.
There are a variety of different signal modulation methods. The chirp sequence modulation (CS modulation) represents a modulation type that is especially frequently used in automotive radar systems. In this type of modulation, a sequence of what is known as chirp signals (linear frequency-modulated, electromagnetic signals) is emitted, for which the instantaneous frequency of the signals is linearly variable in time. One of the advantages of the chirp sequence modulation is that it allows for a simultaneous estimate of a distance and a relative velocity of objects.
After reflected chirp signals have been received and suitably preprocessed, the reflected time signals are typically stored in a two-dimensional matrix. Each column of the matrix includes values of received signals of a chirp, the number of columns of the matrix corresponding to the number of chirp signals of a transmission sequence.
A discrete Fourier transform (DFT) for data elements of a chirp along a column of the data matrix allows for an estimate of a distance (or a distance range) of targets in a coverage zone of the radar. The data elements of the columns of the matrix represent distances of target objects. A performance of a second discrete Fourier transform along the lines of the resulting matrix allows for an estimate of the relative velocity of the target objects.
Carrying out a two-dimensional discrete Fourier transform produces a distance-velocity-power matrix (d-v matrix); the amplitude values of the elements of the d-v matrix in a third dimension represent estimates of the reflected signal energy for the corresponding distance and the corresponding velocity of the target object. In practice, a fast Fourier transform is typically performed for this purpose, which realizes an efficient implementation of the discrete Fourier transform.